Cathode active material for secondary batteries and secondary battery including the same

ABSTRACT

Provided is a cathode active material including a complex coating layer, which includes M below, formed on a surface of the cathode active material through reaction of a lithium transition metal oxide represented by Formula 1 below with a coating precursor: 
       Li x MO 2   (1)
         wherein M is represented by Mn a M′ 1-b , M′ is at least one selected from the group consisting of Al, Mg, Ni, Co, Cr, V, Fe, Cu, Zn, Ti and B, 0.95≦x≦1.5, and 0.5≦a≦1.       

     The lithium secondary battery including the cathode active material exhibits improved lifespan and rate characteristics due to superior stability.

TECHNICAL FIELD

The present invention relates to a cathode active material for secondarybatteries surface-treated and a lithium secondary battery including thesame. More particularly, the present invention relates to a cathodeactive material surface-treated by reacting a lithium transition metaloxide having a specific composition with a coating precursor and therebyforming a complex coating layer including a transition metal derivedfrom the lithium transition metal oxide, and a lithium secondary batteryincluding the same.

BACKGROUND ART

As mobile device technology continues to develop and demand thereforcontinues to increase, demand for secondary batteries as energy sourcesis rapidly increasing. Among these secondary batteries, lithiumsecondary batteries, which have high energy density and operatingvoltage, long cycle lifespan, and low self-discharge rate, arecommercially available and widely used.

In addition, as interest in environmental problems is increasing,research into electric vehicles (EVs) and hybrid EVs (HEVs) that canreplace vehicles using fossil fuels, such as gasoline vehicles, dieselvehicles, and the like, which are one of the main causes of airpollution, is actively underway. As a power source of EVs, HEVs, and thelike, a nickel metal-hydride secondary battery is mainly used. However,research into lithium secondary batteries having high energy density andhigh discharge voltage is actively underway and some lithium secondarybatteries are commercially available.

Lithium ion secondary batteries used in conventional small batteriesgenerally use a lithium cobalt composite oxide having a layeredstructure as a cathode and a graphite material as an anode. However, inthe case of the lithium cobalt composite oxide, cobalt, a mainconstitution element, is very expensive and the lithium cobalt compositeoxide is not sufficiently stable for application to electric vehicles.Therefore, as cathode of a lithium ion battery for electric vehicles, alithium manganese composite oxide composed of manganese, which is cheapand has superior stability, and having a spinel structure may be proper.

However, in the case of the lithium manganese composite oxide, manganeseis eluted to an electrolyte solution by an electrolyte solution duringhigh-temperature storage and, thereby, degrading batterycharacteristics. Therefore, solutions to prevent this problem arerequired. In addition, the lithium manganese composite oxide has adrawback that capacity per unit weight is small, when compared tolithium cobalt composite oxides or lithium nickel composite oxides, andthereby increase of capacity per battery weight is limited. Accordingly,when a battery is designed such that the limitation is improved, thebattery may be commercialized as a power source of electric vehicles.

Such cathode active materials during charging may reduce stability of abattery cell through exothermic reaction accompanying degradation of asurface structure and drastic structural collapse. Thermal stability isassociated with interfacial stability between an electrolyte and acathode active material. Accordingly, most patent literature usesgeneral coating methods to improve surface stability and disclose aplurality of different coating methods.

When the prior art is taken together, there are two type coatingmethods, namely, cathode ion coating and anode ion coating. Al₂O₃coating is a representative example of cathode ion coating and anexample of anode ion coating includes fluoride, phosphate, and silicatecoating. Here, fluoride coating is the most preferable in that thefluoride coating is thermodynamically very stable due to formation of aprotective film of LiF and may provide satisfactory stability at hightemperature and voltage, since the fluoride coating does not react withan electrolyte. Meanwhile, coating type may be classified into inorganiccoating and organic coating, and polymer coating as an example of theorganic coating may provide an elastic coating.

However, the conventional coating methods cannot provide satisfactorybattery cell stability. As well as, thin and dense LiF films cannot beprovided due to a high melting point and poor wetting properties of LiFand polymer coating may deteriorate overall properties of a lithiumsecondary battery due to problems such as poor electrical conductivityand lithium migration.

Therefore, there is an urgent need for coating technology which mayimprove stability by protecting a surface of a cathode active materialwithout deterioration of battery characteristics and improve overallproperties of a battery.

DISCLOSURE Technical Problem

Therefore, the present invention has been made to solve the above andother technical problems that have yet to be resolved.

As a result of a variety of extensive and intensive studies andexperiments, the inventors of the present invention confirmed that, whena cathode active material, in which a predetermined lithium transitionmetal oxide reacts with a coating precursor and thereby a complexcoating layer comprising a transition metal derived from the lithiumtransition metal is formed, is used, surfaces of cathode active materialparticles may be prevented from damage and lithium ion migrationcharacteristics may be improved, and, as such, desired effects may beaccomplished, thus completing the present invention.

Technical Solution

In accordance with one aspect of the present invention, provided is acathode active material including a complex coating layer, whichincludes M below, formed on a surface of the cathode active materialthrough reaction of a lithium transition metal oxide represented byFormula 1 below with a coating precursor:

Li_(x)MO₂  (1)

wherein M is represented by Mn_(a)M′_(1-b), M′ is at least one selectedfrom the group consisting of Al, Mg, Ni, Co, Cr, V, Fe, Cu, Zn, Ti andB, 0.95≦x≦1.5, and 0.5≦a≦1.

Generally, in lithium manganese-based oxides, irreversible capacity lossis great when a battery is charged and discharged, and ratecharacteristics are low due to low electrical conductivity. Suchproblems are deepened with increasing amount of manganese.

The cathode active material according to the present invention includesa predetermined metal coating layer and thereby a surface of a cathodeactive material particles is protected during high-ratecharge/discharge, and, accordingly, stability is improved. In addition,migration characteristics of lithium ions are improved and, as such,reversible discharge capacity and rate characteristics may be improved.

When the amount of the lithium transition metal oxide of Formula 1 isexcessively large or small, discharge capacity may be reduced duringhigh voltage charge/discharge. Preferably, the amount of the lithium maybe 1≦x≦1.5.

A metal of the lithium transition metal oxide according to Formula 1,namely, M is represented by Mn_(a)M′_(1-b) and the amount of themanganese is in a range to exhibit optimal effects. Therefore,excessively low manganese amount is not preferable. Preferably, theamount of the manganese may be 0.6≦a≦0.9. In addition, a metal, namely,M′ may be at least one selected from the group consisting of Ni, Co, Cr,V, Fe, Cu, and B.

The lithium transition metal oxide according to Formula 1 may form acomplex coating layer including a metal, namely, M, derived from thelithium transition metal oxide by reacting with a first coatingprecursor and a second coating precursor at the same time.

Such a complex coating layer, for example, may have at least onecombination structure selected form the group consisting of Li-M-X,Li-M-m and Li-M-X-m, X may be a halogen element derived from the firstcoating precursor, and m may be a metal element derived from the secondcoating precursor.

As an embodiment, the first coating precursor may be an organic orinorganic compound including a halogen element, namely, X. In thisregard, X may be F, Cl, Br or I derived from the organic or inorganiccompound, particularly F or Cl.

The organic or inorganic compound is not specifically limited so long asthe compound includes F, Cl, Br or I. For example, the organic compoundmay be any one selected the group consisting of PVdF, PVdF-HFP, PVF,PTFE and ETFE, and the inorganic compound may be a lithium salt orammonium salt including a halogen element.

More particularly, the first coating precursor may be PVdF including F.

As an embodiment, the second coating precursor may be an oxide includinga metal, namely, m. In this regard, the metal, namely, m, may be atleast one selected from the group consisting of Al, Ba, Ca, Mg, Si, Ti,Zr, Zn, and Sr, derived from the oxide.

The oxide is not limited so long as the oxide includes at least oneselected from Al, Ba, Ca, Mg, Si, Ti, Zr, Zn, and Sr. For example, theoxide may be Al₂O₃.

As another embodiment, the second coating precursor may be acarbonate-based material including the metal, namely, m. In this regard,the metal, namely, m, may be at least one selected from the groupconsisting of Al, Ba, Ca, Mg, Si, Ti, Zr, Zn, and Sr derived from thecarbonate-based material.

The carbonate-based material is not limited so long as thecarbonate-based material includes at least one selected from Al, Ba, Ca,Mg, Si, Ti, Zr, Zn, and Sr. For example, the carbonate-based materialmay be CaCO₃.

As another embodiment, the second coating precursor may be an organicmatter including the metal, namely, m. In this regard, the metal,namely, m, may be at least one selected from the group consisting of Al,Ba, Ca, Mg, Si, Ti, Zr, Zn, and Sr derived from the organic matter.

Generally, the organic matter includes C and H, selectively O and thelike. The organic matter according to the present invention may furtherinclude at least one selected from Al, Ba, Ca, Mg, Si, Ti, Zr, Zn, andSr, particularly Al, more particularly C₉H₂₁O₃Al.

As a specific embodiment, in a complex coating layer having thecombination structure described above, the amount of X may be 0.01 to1.00 wt %, particularly 0.15 to 0.5 wt % based on the total weight ofthe cathode active material. In addition, the amount of m may be 0.01 to0.5 wt %, particularly 0.05 to 0.3 wt %, based on the total weight ofthe cathode active material.

In the complex coating layer, when the amounts of X and m areexcessively large, the amount of the lithium transition metal oxide isrelatively reduced and thereby desired capacity may not be obtained. Onthe other hand, when the amounts of X and m are excessively small,desired battery cell stability improvement effects may not be obtained.

A process of preparing such a cathode active material, for example,includes: (a) preparing the lithium transition metal oxide according toFormula 1; (b) coating the first coating precursor and the secondcoating precursor on the lithium transition metal oxide at the sametime; and (c) heat-treating after the coating according to step (b).

A composition formula of the lithium transition metal oxide according toFormula 1, a first coating precursor, and a second coating precursor arethe same as described above.

The heat-treatment may be performed, for example, at a temperature rangeof 250 to 600° C.

The coating of step (b) may be performed using a dry method to preventdeterioration of cathode active material characteristics.

In a specific embodiment, the complex coating layer may be formed on aportion of a surface of the lithium transition metal oxide in a spotform when the high energy milling or dry method-based mixing is used.

Such a coating method is broadly known in the art and thus descriptionthereof is omitted.

The coating of step (b) may be performed using the dry method to preventdeterioration of cathode active material characteristics.

In addition, in step (b), the first coating precursor and the secondcoating precursor may be used in a mixing ratio of 2:3 to 5:2.

The present invention also provides a cathode mixture for secondarybatteries including the cathode active material described above and acathode for secondary batteries including the cathode mixture.

The cathode mixture may selectively include, in addition to the cathodeactive material, a conductive material, a binder, a filler and the like.

The conductive material is typically added in an amount of 1 to 30 wt %based on a total weight of a mixture including a cathode activematerial. There is no particular limit as to the conductive material, solong as it does not cause chemical changes in the fabricated battery andhas conductivity. Examples of conductive materials include, but are notlimited to, graphite such as natural or artificial graphite; carbonblack such as carbon black, acetylene black, Ketjen black, channelblack, furnace black, lamp black, and thermal black; conductive fiberssuch as carbon fibers and metallic fibers; metallic powders such ascarbon fluoride powder, aluminum powder, and nickel powder; conductivewhiskers such as zinc oxide and potassium titanate; conductive metaloxides such as titanium oxide; and polyphenylene derivatives.

The binder is a component assisting in binding between an activematerial and a conductive material and in binding of the active materialto a current collector. The binder may be typically added in an amountof 1 wt % to 30 wt % based on a total weight of a mixture including acathode active material. Examples of the binder include, but are notlimited to, polyvinylidene fluoride, polyvinyl alcohols,carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene,polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM),sulfonated EPDM, styrene-butadiene rubber, fluorine rubber, and variouscopolymers.

The filler is optionally used as a component to inhibit cathodeexpansion. The filler is not particularly limited so long as it is afibrous material that does not cause chemical changes in the fabricatedsecondary battery. Examples of the filler include olefin-based polymerssuch as polyethylene and polypropylene; and fibrous materials such asglass fiber and carbon fiber.

The cathode according to the present invention may be prepared bycoating and then pressing a slurry, which is prepared by mixing acathode mixture including the compounds described above with a solventsuch as NMP and the like, on a cathode collector.

The cathode current collector is generally fabricated to a thickness of3 to 500 μm. The cathode current collector is not particularly limitedso long as it does not cause chemical changes in the fabricated batteryand has conductivity. For example, the cathode current collector may bemade of stainless steel, aluminum, nickel, titanium, sintered carbon, oraluminum or stainless steel surface-treated with carbon, nickel,titanium, silver, or the like. The cathode current collector may havefine irregularities at a surface thereof to increase adhesion betweenthe cathode active material and the cathode current collector. Inaddition, the cathode current collector may be used in any of variousforms including films, sheets, foils, nets, porous structures, foams,and non-woven fabrics.

In addition, the present invention provides a lithium secondary batterycomposed of the cathode, an anode, a separator, and a lithiumsalt-containing non-aqueous electrolyte solution.

The anode, for example, is prepared by coating and then drying an anodemixture including an anode active material on an anode collector. Asdesired, the anode mixture may include ingredients as described above.

Examples of the anode active material include, without being limited to,carbon such as hard carbon and graphite-based carbon; metal compositeoxides such as Li_(x)Fe₂O₃ where 0≦x≦1, Li_(x)WO₂ where 0≦x≦1,Sn_(x)Me_(1-x)Me′_(y)O_(z) where Me: Mn, Fe, Pb, or Ge; Me′: Al, B, P,Si, Groups I, II and III elements, or halogens; 0≦x≦1; 1≦y≦3; and 1≦z≦8;lithium metals; lithium alloys; silicon-based alloys; tin-based alloys;metal oxides such as SnO, SnO₂, PbO, PbO₂, Pb₂O₃, Pb₃O₄, Sb₂O₃, Sb₂O₄,Sb₂O₅, GeO, GeO₂, Bi₂O₃, Bi₂O₄, Bi₂O₅, and the like; conductive polymerssuch as polyacetylene; and Li—Co—Ni-based materials.

The anode current collector is typically fabricated to a thickness of 3to 500 μm. The anode current collector is not particularly limited solong as it does not cause chemical changes in the fabricated battery andhas conductivity. For example, the anode current collector may be madeof copper, stainless steel, aluminum, nickel, titanium, sintered carbon,copper or stainless steel surface-treated with carbon, nickel, titanium,or silver, and aluminum-cadmium alloys. As in the cathode currentcollector, the anode current collector may have fine irregularities at asurface thereof to enhance adhesion between the anode current collectorand the anode active material. In addition, the anode current collectormay be used in various forms including films, sheets, foils, nets,porous structures, foams, and non-woven fabrics.

The separator is disposed between the cathode and the anode and aninsulating thin film having high ion permeability and mechanicalstrength is used as the separator. The separator typically has a porediameter of 0.01 to 10 μm and a thickness of 5 to 300 μm. As theseparator, sheets or non-woven fabrics made of an olefin polymer such aspolypropylene, glass fibers or polyethylene, which have chemicalresistance and hydrophobicity, are used. When a solid electrolyte suchas a polymer is used as the electrolyte, the solid electrolyte may alsoserve as a separator.

The lithium salt-containing non-aqueous electrolyte solution is composedof an electrolyte solution and a lithium salt. As the electrolytesolution, a non-aqueous inorganic solvent, organic solid electrolyte,inorganic solid electrolyte, and the like are used.

For example, the non-aqueous organic solvent may be an aprotic organicsolvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylenecarbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate,gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trimethoxy methane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate, ethyl propionate,or the like.

Examples of the organic solid electrolyte include polyethylenederivatives, polyethylene oxide derivatives, polypropylene oxidederivatives, phosphoric acid ester polymers, poly agitation lysine,polyester sulfide, polyvinyl alcohols, polyvinylidene fluoride, andpolymers containing ionic dissociation groups.

Examples of the inorganic solid electrolyte include nitrides, halidesand sulfates of lithium (Li) such as Li₃N, LiI, Li₅NI₂, Li₃N—LiI—LiOH,LiSiO₄, LiSiO₄—LiI—LiOH, Li₂SiS₃, Li₄SiO₄, Li₄SiO₄—LiI—LiOH, andLi₃PO₄—Li₂S—SiS₂.

The lithium salt is a material that is readily soluble in thenon-aqueous electrolyte. Examples thereof include LiCl, LiBr, LiI,LiClO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆,LiAlCl₄, CH₃SO₃Li, CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroborane lithium, loweraliphatic carboxylic acid lithium, lithium tetraphenyl borate, andimide. Among these, LiPF₆ is the most preferable.

In addition, in order to improve charge/discharge characteristics andflame retardancy, for example, pyridine, triethylphosphite,triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphorictriamide, nitrobenzene derivatives, sulfur, quinone imine dyes,N-substituted oxazolidinone, N,N-substituted imidazolidine, ethyleneglycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol,aluminum trichloride, or the like may be added to the non-aqueouselectrolyte solution. In some cases, in order to impartincombustibility, the electrolyte may further include ahalogen-containing solvent such as carbon tetrachloride and ethylenetrifluoride. In addition, in order to improve high-temperature storagecharacteristics, the electrolyte may further include carbon dioxide gas,fluoro-ethylene carbonate (FEC), propene sultone (PRS), fluoro-propylenecarbonate (FPC) or the like.

The secondary battery according to the present invention may be used ina battery cell used as a power source of small devices and may also beused as a unit cell of a medium and large-scale battery module includinga plurality of battery cells.

The present invention also provides a battery pack including the batterymodule as a power source of a medium and large-scale device. Examples ofthe medium and large-scale device include, but are not limited to,electric vehicles (EVs), hybrid EVs (HEVs), and plug-in HEVs (PHEVs);and devices for storing power.

Effects of Invention

As described above, in a cathode active material according to thepresent invention, a predetermined lithium transition metal oxide formsa complex coating layer including a transition metal derived from alithium transition metal and thereby the coating layer improvesstability by protecting a surface of a cathode active material underhigh voltage, and, accordingly, lifespan characteristics of a secondarybattery including the same may be improved. In addition, migrationcharacteristics of lithium ions are improved and, as such, reversibledischarge capacity may be increased and rate characteristics may beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanying drawing,in which:

FIG. 1 is a scanning electron microscope (SEM) image of a cathode activematerial according to Example 1;

FIG. 2 is an SEM image of a cathode active material according toComparative Example 1;

FIG. 3 is a graph illustrating discharge capacity according toExperimental Example 2; and

FIG. 4 is a graph illustrating discharge capacity according toExperimental Example 3.

MODE FOR INVENTION

Now, the present invention will be described in more detail withreference to the following examples. These examples are provided onlyfor illustration of the present invention and should not be construed aslimiting the scope and spirit of the present invention.

Example 1

Li_(1.11)Mn_(0.53)Ni_(0.36)O₂ powder having a particle size of 5 to 10μm was prepared. In addition, NH₄F and C₉H₁₂O₃Al were mixed in a massratio of 1:1.18 and then mixed with Li_(1.11)Mn_(0.53)Ni_(0.36)O₂powder. In the Li_(1.11)Mn_(0.53)Ni_(0.36)O₂ powder, the amount of F is0.08 wt % and the amount of Al is 0.04 wt %. Subsequently, the resultingmixture was heat-treated at 400° C. for 5 hours, and then was pulverizedand sieved.

Comparative Example 1

Li_(1.11)Mn_(0.53)Ni_(0.36)O₂ powder having a particle size of 5 to 10μm was prepared.

Experimental Example 1

SEM images of cathode active materials prepared according to Example 1and Comparative Example 1 are illustrated FIGS. 1 and 2, respectively.

As shown in FIG. 1, Li-M-F, Li-M-Al, and Li-M-F—Al coating layers formedon a surface of the cathode active material according to Example 1 maybe observed.

Experimental Example 2

The cathode active material prepared according to each of Example 1 andComparative Example 1 was mixed in NMP such that a ratio (wt %) ofactive material:conductive material:binder was 95:2.5:2.5. Subsequently,the resulting mixtures were coated over aluminum (Al) foil having athickness of 20 μm and then dried at 130° C., resulting in cathodeelectrodes. Graphite as an anode active material and a solvent composedof EC, DMC, and DEC mixed in a ratio of 1:2:1 including 1M LiPF₆ as anelectrolyte solution were used, resulting in batteries. The resultingbatteries were charged and discharged at 2.5 to 4.65 V and 0.1 C, andresulting discharge capacities are shown in Table 1 below and FIG. 3.

TABLE 1 Example 1 Comparative Example 1 (after coating) (before coating)Discharge capacity at 0.1 C 219 mAh/g 213 mAh/g

Experimental Example 3

The batteries manufactured according to Experimental Example 2 weretested by charging and discharging at 3.0 to 4.4 V and at 0.5 C. Here,lifespan characteristics were estimated by a maintenance ratio withrespect to initial capacity after 30 cycles. Results are summarized inTable 2 below and FIG. 4.

TABLE 2 Comparative Example 1 Example 1 (after coating) (before coating)Discharge capacity at 0.5 C 153.3 mAh/g 148.3 mAh/g (lifespancharacteristics) (90.8%) (90.1%)

Experimental Example 4

Regarding the batteries manufactured according to Experimental Example2, capacity according to each C-rate with respect to capacity at 0.1 Cwas calculated by testing rate characteristics at 3.0 to 4.4 V. Resultsare summarized in Table 3 below.

TABLE 3 Example 1 Comparative Example 1 (after coating) (before coating)0.1 C 186.6 mAh/g (100%)  182.6 mAh/g (100%)  0.5 C 159.6 mAh/g (85.5%)153.9 mAh/g (84.3%) 1.0 C 145.9 mAh/g (78.2%)   138 mAh/g (75.6%)

As shown in Tables 1 to 3 and FIGS. 2 to 4, due to Li-M-F, Li-M-Al, andLi-M-F—Al layers formed over the surface of the cathode active materialaccording to Example 1, lifespan and capacity characteristics of thebattery using the cathode active material are superior, when compared tothe battery using the cathode active material according to ComparativeExample 1.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A cathode active material comprising a complex coating layer,comprising M below, formed over a surface of the cathode active materialthrough reaction of a lithium transition metal oxide represented byFormula 1 below with a coating precursor:Li_(x)MO₂  (1) wherein M is represented by Mn_(a)M′_(1-b), M′ is atleast one selected from the group consisting of Al, Mg, Ni, Co, Cr, V,Fe, Cu, Zn, Ti and B, and 0.95≦x≦1.5 and 0.5≦a≦1.
 2. The cathode activematerial according to claim 1, wherein the complex coating layer has atleast one combination structure selected from the group consisting ofLi-M-X, Li-M-m and Li-M-X-m, X is a halogen element derived from a firstcoating precursor, and m is a metal element derived from a secondcoating precursor.
 3. The cathode active material according to claim 2,wherein the first coating precursor is an organic or inorganic compoundcomprising a halogen element, X, and X is F, Cl, Br or I derived fromthe organic or inorganic compound.
 4. The cathode active materialaccording to claim 3, wherein the organic compound is any one selectedfrom the group consisting of PVdF, PVdF-HFP, PVF, PTFE and ETFE, and theinorganic compound is a lithium salt or an ammonium salt comprising ahalogen element.
 5. The cathode active material according to claim 3,wherein the first coating precursor is PVdF.
 6. The cathode activematerial according to claim 2, wherein the second coating precursor isan oxide comprising a metal, m, and the metal, m, is at least oneselected from the group consisting of Al, Ba, Ca, Mg, Si, Ti, Zr, Zn,and Sr derived from the oxide.
 7. The cathode active material accordingto claim 6, wherein the second coating precursor is Al₂O₃.
 8. Thecathode active material according to claim 2, wherein the second coatingprecursor is a carbonate-based material comprising the metal, m, and themetal, m, is at least one selected from the group consisting of Al, Ba,Ca, Mg, Si, Ti, Zr, Zn, and Sr derived from the carbonate-basedmaterial.
 9. The cathode active material according to claim 8, whereinthe second coating precursor is CaCO₃.
 10. The cathode active materialaccording to claim 2, wherein the second coating precursor is an organicmatter comprising the metal, m, and the metal, m, is at least oneselected from the group consisting of Al, Ba, Ca, Mg, Si, Ti, Zr, Zn,and Sr derived from the organic matter.
 11. The cathode active materialaccording to claim 10, wherein the second coating precursor isC₉H₁₂O₃Al.
 12. The cathode active material according to claim 2, whereinthe complex coating layer is formed in a 60 to 100% coating area basedon a surface area of the lithium transition metal oxide.
 13. The cathodeactive material according to claim 2, wherein, in the complex coatinglayer, an amount of X is 0.01 to 1.00 wt % based on a total weight ofthe cathode active material, and an amount of m is 0.01 to 0.50 wt %based on a total weight of the cathode active material.
 14. A method ofpreparing the cathode active material according to claim 1 comprising:preparing a lithium transition metal oxide according to Formula 1;coating a first coating precursor and a second coating precursor at thesame time over the lithium transition metal oxide; and heat-treatingafter the preparing and the coating.
 15. The method according to claim14, wherein the coating is formed by a high energy milling or a drymethod-based mixing.
 16. The method according to claim 14, wherein, inthe coating, the first coating precursor and the second coatingprecursor are used in a mixing ratio of 2:3 to 5:2 by weight.
 17. Acathode mixture comprising the cathode active material according toclaim
 1. 18. A cathode for secondary batteries cathode, wherein thecathode mixture according to claim 17 is coated over a collector.
 19. Asecondary battery comprising the cathode for the secondary batteriesaccording to claim
 18. 20. The secondary battery according to claim 19,wherein the secondary battery is a lithium secondary battery.